FIG. 1 is a circuit diagram of a conventional LED display system 10, in which a charge pump 12 is configured to convert an input voltage VIN into an output voltage VOUT supplied for LEDs D1 to DN, each of the LEDs D1-DN is connected with a respective current regulator 16 to control the driving current ILEDj (j=1, . . . , N) thereof, the current regulator 16 includes an operational amplifier 18 having two input terminals connected to nodes N1 and N2 respectively, a transistors M1 connected between the node N1 and a ground terminal GND to receive a reference current IREFj (j=1, . . . , N), and a transistors M2 connected between the node N2 and the ground terminal GND to establish the driving current ILEDj it controls, the operational amplifier 18 has an output terminal connected to the gates of the transistors M1 and M2 and maintains the voltages at the nodes N1 and N2 to be substantially equal to each other, so that the driving current ILEDj will reflect the reference current IREFj in proportion, depending on the size ratio of the transistors M2 to M1, and a mode decision circuit 14 is configured to detect the voltages VDS1 to VDSN at the nodes N2 of all the current regulators 16 to determine an operation mode of the charge pump 12.
FIG. 2 is a diagram showing a relationship between the input voltage VIN and efficiency of the LED display system 10. As shown in FIG. 2, the LED display system 10 has the highest efficiency when it operates in a mode x1. When the input voltage VIN decreases such that the mode decision circuit 14 detects the voltage at the node N2 of anyone of the current regulators 16 lower than a threshold, the mode decision circuit 14 signals the charge pump 12 to switch from the mode x1 to a higher mode x1.5. Actually, the LEDs D1-DN will not have a same forward voltage and therefore when the voltage at the node N2 of one of the current regulators 16 is lower than the threshold, the voltages at the nodes N2 of the other current regulators 16 may not be lower than the threshold. In other words, some of the current regulators 16 may still be capable of operating normally in the mode x1. In this case, if the charge pump 12 switches from the mode x1 to the mode x1.5, the LED display system 10 will have efficiency loss.
Therefore, it is desired a circuit and method for efficiency improvement of a LED display system.